Light beam modulators employing color separating mirrors are known. For instance, Balzers AG, Liechtenstein, discloses a broadband printer for film printing wherein three dichroic mirrors are placed in an incoming achromatic light beam to split the beam into 3 channels, one for each primary color. A diaphragm is mounted in each channel to control the intensity of the beam by means of a controlled aperture. The exiting light beams from the various channels are recombined into a single outgoing light beam by three more dichroic mirrors placed in the paths of the respective exiting light beams.
This type of modulator operates by sequential formation of color and lacks the capability continuously to regulate color in the outgoing beam; resultant color can be only of a primary or mixture of primary colors. The response time of such a modulator and its reliability of operation are limited by the mechanical or electromechanical components of the diaphragm, and time varying video data projection is not possible. Additionally, since the mechanical diaphragms absorb the light which does not pass through the aperture, the modulator can exhibit heating.
A sequential coloring display employing a liquid crystal cell is disclosed in British patent application GB 2 162 356. Achromatic light is passed through a dichroic polarizer to an electrically switchable liquid crystal cell, so that, in the absence of a field, one component of polarized incident light is rotated 90.degree. by the cell, or, in the presence of a field, is transmitted unrotated. The liquid crystal cell is of the pi-type; a twist-cell-type liquid crystal cell may also be employed. The light finally emerging from the cell is passed through a neutral polarizer. The sequential character of the coloring procedure as well as the use of polarizers result in a high loss of light, only about 10% of the incoming achromatic light being transmitted.